1. Field of the Invention
This invention relates to a display device, and more particularly to a poly-silicon liquid crystal display device with an improved aperture ratio and a simplified method of fabricating the same.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) device, which includes a plurality of liquid crystal cells in a matrix configuration in a liquid crystal display panel, displays images by controlling the transmittance of light in accordance with video signals. In each liquid crystal cell, a thin film transistor (TFT) is used as a switching device to independently supply a video signal. An active layer of such a TFT is generally formed of either amorphous silicon or polycrystalline silicon (poly-silicon). Because the carrier mobility of poly-silicon is approximately hundred times faster than the carrier mobility of amorphous silicon, high-speed driving circuits can be integrally formed in the LCD panel with the poly-silicon technology.
A poly-silicon LCD device generally includes a TFT substrate provided with driving circuits and a color filter substrate provided with a color filter, with liquid crystal provided therebetween.
FIG. 1 is a plan view illustrating a portion of a TFT substrate in a poly-silicon LCD device according to the related art, and FIG. 2 is a cross-sectional view of the TFT substrate taken along the line I-I′ in FIG. 1.
Referring to FIGS. 1 and 2, the TFT substrate includes a thin film transistor (TFT) 30 connected to a gate line 2 and a data line 4, and a pixel electrode 22 connected to the TFT 30. Although either an NMOS-TFT or PMOS-TFT can be used for the TFT 30, the TFT 30 employing an NMOS-TFT will now be described.
The TFT 30 has a gate electrode 6 connected to the gate line 2, a source electrode connected to the data line 4, and a drain electrode 10 connected to the pixel electrode 22 via a pixel contact hole 20 passing through a protective film 18. The gate electrode 6 overlaps a channel area 14C of an active layer 14 provided on a buffer film 12 with a gate insulating film 16 therebetween. The source electrode and the drain electrode 10 are formed in such a manner to be insulated from the gate electrode 6 with an interlayer insulating film 26 therebetween. Further, the source electrode and the drain electrode 10 are connected to a source area 14S and a drain area 14D of the active layer 14 doped with an n+ impurity, respectively, via a source contact hole 24S and a drain contact hole 24D passing through the interlayer insulating film 26 and the gate insulating film 16.
Such a TFT substrate of a poly-silicon LCD device can be fabricated by a six-mask process which is illustrated in FIG. 3A to FIG. 3F.
Referring to FIG. 3A, the buffer film 12 is formed on a lower substrate 1 and then the active layer 14 is formed on the buffer film 12 by a first mask process. The active layer 14 is formed by depositing an amorphous silicon layer on the buffer film 12 and then crystallizing it into a poly-silicon layer using a laser, and thereafter by patterning it with photolithography and etching processes using a first mask.
Referring to FIG. 3B, the gate insulating film 16 is formed on the buffer film 12 provided with the active layer 14 and then the gate line 2 and the gate electrode 6 are formed thereon by a second mask process. Then, an n+ impurity is doped into a non-overlapping area of the active layer 14 using the gate electrode 6 as a mask, thereby forming a source area 14S and a drain area 14D of the active layer 14.
Referring to FIG. 3C, the interlayer insulating film 26 is formed on the gate insulating film 16 provided with the gate line 2 and the gate electrode 6, and then source and drain contact holes 24S and 24D passing through the interlayer insulating film 26 and the gate insulating film 16 are formed by a third mask process.
Referring to FIG. 3D, the data line 4 including the source electrode and the drain electrode 10 are formed on the interlayer insulating film 26 by a fourth mask process.
Referring to FIG. 3E, the protective film 18 is formed on the interlayer insulating film 26 provided with the data line 4 and the drain electrode 10, and then the pixel contact hole 20 passing through the protective film 18 is formed by a fifth mask process to expose the drain electrode 10.
Referring to FIG. 3F, the transparent pixel electrode 22 is formed on the protective film 18 by a sixth mask process.
As described above, the TFT substrate of the related art is formed by a six-mask process. Because each mask process includes many sub-processes such as deposition, cleaning, photolithography, etching, photo-resist stripping and inspection, etc., the manufacturing process is complicated and the manufacturing cost is high.